cross train and lu/qna build optimization for composer

[feich-ms]

fix #883, #880, #879, #878 and microsoft/botbuilder-dotnet#4144. All these issues are related with composer crosstrain and lu/qna:build scenarios. This PR is to resolve these issues including:

1. add filter meta data for all qna pairs
2. adjust the recognizer generation logic for qna to generate recognizer dialog files for each dialog instead of sharing single one of botName, but in settings file, the kb ids of qna recognizers will be same for same locale.
3. Optimize lu/qna builder interface and flow to write out the empty crosstrained recognizer for both lu and qna files are empty scenario
4. Do not add any crosstrained contents to lu or qna files if all lu files or all qna files are empty in crosstrained flow
5. adjust and add Unit tests

Now composer only need to call below flow directly and DO NOT need any other tricy calling logic:
call crosstrain() -> lubuild.loadContents() ->lubuild.build() -> lubuild.writeDialogAssets() -> qnabuild.loadContents() -> qnabuild.build() -> qnabuild.writeDialogAssets()

[codecov-commenter]

Codecov Report

Merging #889 into master will decrease coverage by 2.92%.
The diff coverage is 20.73%.

@@            Coverage Diff             @@
##           master     #889      +/-   ##
==========================================
- Coverage   60.45%   57.53%   -2.93%     
==========================================
  Files         220      219       -1     
  Lines       17887    16503    -1384     
  Branches     2316     2265      -51     
==========================================
- Hits        10814     9495    -1319     
+ Misses       6494     6428      -66     
- Partials      579      580       +1     

Impacted Files	Coverage Δ
packages/lu/src/parser/lu/luMerger.js	76.34% <0.00%> (-3.77%)	⬇️
packages/lu/src/parser/lubuild/builder.ts	11.83% <0.00%> (-0.98%)	⬇️
packages/lu/src/parser/lubuild/core.ts	15.96% <ø> (-0.34%)	⬇️
packages/lu/src/parser/utils/enums/CLI-errors.js	100.00% <ø> (ø)
packages/lu/src/parser/qnabuild/builder.ts	19.86% <3.73%> (-2.87%)	⬇️
packages/lu/src/parser/qnabuild/core.ts	23.52% <50.00%> (+1.20%)	⬆️
packages/lu/src/utils/filehelper.ts	40.88% <50.00%> (-9.35%)	⬇️
packages/qnamaker/src/commands/qnamaker/build.ts	77.00% <83.33%> (+0.15%)	⬆️
packages/lu/src/parser/cross-train/crossTrainer.js	92.43% <97.61%> (-2.29%)	⬇️
packages/luis/src/commands/luis/build.ts	79.56% <100.00%> (-0.44%)	⬇️
... and 104 more

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[boydc2014]

@alanlong9278 @luhan2017 can you help verify this feature is as what we expected at composer side